If geolocation is the much-hyped next frontier of social networking, Naveen Selvadurai is at its nexus. Alongside partner Dennis Crowley, the Indian native runs Foursquare, a smartphone app that gives users virtual points and badges for "checking in" at physical venues. In just 14 months, he's helped his New York-based startup rack up almost a million users. "We're trying to make people better at navigating their cities," says Selvadurai, 28. Foursquare has proved magnetic to big-name brands eager to leverage location-based advertising. HBO, Bravo, and Zagat have already inked deals allowing Foursquare users to earn rewards for checking into the restaurants and retailers they feature.